Electronically-controlled coupling for all-wheel drive system

ABSTRACT

A drive axle for an all-wheel drive vehicle includes an adaptively controlled first hydraulic coupling for providing front-to-rear torque transfer control and a passively controlled second hydraulic coupling for providing side-to-side limited slip and torque biasing control. The drive axle is contained with a common housing and communicates with a tractor control system to actively control actuation of the first hydraulic coupling based on the operating characteristics of the vehicle as detected by suitable sensors.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/281,888, filed Apr. 5, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates generally to hydraulic couplingsfor use in motor vehicle driveline applications for limiting slip andtransferring torque between rotary members. More specifically, a driveaxle assembly for an all-wheel drive vehicle is disclosed having a pairof hydraulic couplings each having a fluid pump, a multi-plate clutchassembly, and a fluid distribution system operable to control actuationof the clutch assembly.

BACKGROUND OF THE INVENTION

[0003] In all-wheel drive vehicles, it is common to have a secondarydrive axle that automatically receives drive torque from the drivetrainin response to lost traction at the primary drive axle. In suchsecondary drive axles, it is known to provide a pair of clutchassemblies connecting each axleshaft to a prop shaft that is driven bythe drivetrain. For example, U.S. Pat. No. 4,650,028 discloses asecondary drive axle equipped with a pair of viscous couplings. Inaddition, U.S. Pat. Nos. 5,964,126, 6,095,939, 6,155,947 and 6,186,258each disclose secondary drive axles equipped with a pair ofpump-actuated multi-plate clutch assemblies. In contrast to thesepassively-controlled secondary drive axles, U.S. Pat. No. 5,699,888teaches of a secondary drive axle having a pair of multi-plate clutchesthat are actuated by electromagnetic actuators that are controlled by anelectronic control system.

[0004] In response to increased consumer demand for motor vehicles withtraction control systems, hydraulic couplings are currently being usedin a variety of driveline applications. Such hydraulic couplings rely onhydromechanics and pressure-sensitive valve elements to passivelyrespond to a limited range of vehicle operating conditions. Thesehydraulic couplings are susceptible to improvements that enhance theirperformance, such as a more controlled response to a wider range ofvehicle operating conditions. With this in mind, a need exists todevelop improved hydraulic couplings that advance the art.

SUMMARY OF THE INVENTION

[0005] Accordingly, the present invention provides a hydraulic couplingfor use in motor vehicle driveline applications for rotatively couplinga pair of rotary members to limit speed differentiation and transferdrive torque therebetween.

[0006] Another object of the present invention is to provide a driveaxle assembly equipped with a pair of hydraulic couplings which areoperably arranged for coupling a vehicle drivetrain to a pair ofaxleshafts.

[0007] In carrying out the above object, the drive axle assembly of thepresent invention includes a first hydraulic coupling operably disposedbetween the prop shaft and the pinion shaft, and a second hydrauliccoupling installed in a differential drive module. The differentialdrive module includes a drive case driven by the pinion shaft, abevel-type differential unit interconnected between the drive case and apair of axleshafts, and the second hydraulic coupling is operablydisposed between the drive case and at least one of the axleshafts.

[0008] The first hydraulic coupling generally includes a multi-plateclutch assembly operatively connecting a pair of rotary members, anactuator assembly for actuating the clutch assembly, and a fluid controlsystem operable for controlling actuation of the actuator assembly. Theactuator assembly includes a hydraulic pump and a piston mounted in apiston chamber for movement relative to the multi-plate clutch assembly.The fluid control system regulates the fluid pressure supplied to thepiston chamber by the hydraulic pump to control the clutch engagementforce exerted by the piston on the clutch assembly. The fluid controlsystem includes an electrically-controlled flow control valve operablefor regulating the fluid pressure delivered to the piston chamber.Preferably, the flow control valve is a pulse-width modulated (PWM)valve having a moveable valve element. The position of the valve elementis controlled by an electronic traction control module that monitors andresponds to certain vehicle operating conditions including, withoutlimitation, a sump fluid temperature, a coupling outlet oil temperature,the four wheel speeds, and the piston chamber pressure. The electronictraction control module sends a control signal to the PWM control valvefor modulating the hydraulic pressure supplied to the piston chamber,which, in turn, controls clutch engagement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Further objects, features and advantages of the present inventionwill become readily apparent from the following detailed specificationand the appended claims which, in conjunction with the drawings, setforth the best mode now contemplated for carrying out the invention.Referring to the drawings:

[0010]FIG. 1 is a schematic view of a motor vehicle drivetrain equippedwith a secondary drive axle assembly constructed in accordance with thepresent invention;

[0011]FIG. 2 is a sectional view of the secondary drive axle assembly ofthe present invention;

[0012]FIG. 3 is a sectional view of an on-demand hydraulic couplingassociated with the secondary drive axle assembly;

[0013]FIG. 4 is an enlarged partial view taken from FIG. 3 showingcomponents of the hydraulic coupling in greater detail;

[0014]FIG. 5 and 6 are schematic diagrams illustrating a hydrauliccontrol circuit associated with the on-demand hydraulic coupling shownin FIG. 3;

[0015] FIGS. 7 is a sectional view of a differential drive moduleassociated with the secondary drive axle of the present invention; and

[0016]FIG. 8 is a schematic diagram illustrating a hydraulic circuit foran on-demand hydraulic coupling equipped with a variable displacementpump and a torque limiting feature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In general, the present invention is directed to ahydromechanical limited slip and torque transfer device, hereinafterreferred to as a drive axle assembly, for use in connecting thedrivetrain to a pair of axleshafts associated with a secondary drivelineof an all-wheel drive vehicle. However, the drive axle assembly can alsofind application in other driveline applications including, but notlimited to, limited slip differentials of the type used in full-timetransfer cases and front-wheel drive transaxles. Furthermore, thisinvention advances the technology in the field of actively-controlledhydraulically-actuated couplings of the type requiring pressure reliefand thermal unloading to prevent damage to the driveline components.

[0018] With reference to FIG. 1, a schematic layout for a vehiculardrivetrain 10 is shown to include a powertrain 12 driving a first orprimary driveline 14 and a second or secondary driveline 16. Powertrain12 includes an engine 18 and a transaxle 20 arranged to provide motivepower (i.e., drive torque) through a front differential (not shown) to apair of wheels 22 associated with primary driveline 14. In particular,primary driveline 14 includes a pair of halfshafts 24 connecting wheels22 to the front differential associated with transaxle 20. Secondarydriveline 16 includes a power take-off unit (PTU) 26 driven by transaxle20, a prop shaft 28 driven by PTU 26, a pair of axleshafts 30 connectedto a pair of wheels 32, and a drive axle assembly 34 operable totransfer drive torque from propshaft 28 to one or both axleshafts 30.

[0019] Referring to FIGS. 2 through 7, the components associated withdrive axle assembly 34 will be now detailed. Drive axle assembly 34includes a multi-piece housing 40, an input shaft 42, a first hydrauliccoupling 44, and a rear differential drive module 46. Input shaft 42 isrotatably supported in housing 40 by a bearing assembly 48 and sealedrelative thereto via a seal assembly 50. A yoke 52 is secured to inputshaft 42 and is adapted for connection to propshaft 28. Drive module 46includes a pinion shaft 54, a bevel-type differential gearset 56, a pairof output shafts 58 and 60 adapted for connection to axleshafts 30, anda second hydraulic coupling 62. In operation, first hydraulic coupling44 is operable to transfer drive torque from input shaft 42 to pinionshaft 54 in response to excessive interaxle speed differentiationbetween propshaft 28 and differential gearset 56. Second hydrauliccoupling 62 is operable to limit intra-axle slip in response toexcessive speed differentiation between output shafts 58 and 60.

[0020] Shafts 42 and 54 are rotatable relative to one another, withinput shaft 42 being supported by a bearing assembly 64 for rotationrelative to pinion shaft 54. Bearing assemblies 66 are also provided forsupporting pinion shaft 54 for rotation relative to housing 40. As willbecome apparent, hydraulic coupling 44 is controlled by an electronictraction control module 68 for automatically controlling torque transferand speed differentiation between shafts 42 and 54. Electronic tractioncontrol module 68 monitors vehicle system information (i.e., via vehiclesensors 69) and hydraulic coupling information (i.e., via couplingsensors 70). Coupling sensors 70 may include, but are not limited to,wheel speed, oil sump temperature, oil outlet temperature, and clutchpressure. Vehicle sensors 69 may include, but are not limited to, wheelspeed sensors, brake status sensor, transmission gear sensor, vehiclespeed sensor, etc. Control module 68 is operable to control apulse-width modulated (PWM) flow control valve assembly 72 associatedwith hydraulic coupling 44.

[0021] In general, hydraulic coupling 44 comprises two portions: anactuator assembly 74, and a transfer clutch 76 for transferring drivetorque from a faster rotating shaft to a slower rotating shaft inresponse to excessive speed differentiation therebetween. Transferclutch 76 is a hydraulically-actuated multiplate clutch assemblyoperably coupled between input shaft 42 and pinion shaft 54. Actuatorassembly 74 includes a hydraulic pump 78 and a piston assembly 80.Hydraulic pump 78 is confined within a cover assembly 82 which includesa cylindrical outer drum 84 and a cover plate 86 secured via fasteners88 thereto. Cover assembly 82 is fixed for rotation with input shaft 42and, in the embodiment shown, outer drum 84 is integral with input shaft42. Preferably, hydraulic pump 78 is a bidirectional gerotor pump havinga first toothed pump member 90 fixed (i.e., splined) for rotation withpinion shaft 54, and a second toothed pump member 92 journalled in aneccentric chamber formed in outer drum 84. With such an arrangement,relative rotation between input shaft 42 and pinion shaft 54 results ina pumping action which draws fluid from one of a pair of inlet chambers94 on the suction side of pump 78 to a corresponding outlet chamber 96on the discharge side of pump 78. To facilitate pumping action in bothdirections of rotation, hydraulic pump 78 includes suitable one-waycheck valves similar to the arrangement shown in commonly-owned U.S.Pat. No. 6,041,903 which is incorporated by reference. Specifically, apair of one-way check valves 98 are installed in the hydraulic circuitbetween a fluid sump 100 and inlet chambers 94 to maintain a supply offluid therein when pump 78 is static. Likewise, a second pair of checkvalves 102 are located in the fluid circuit between outlet chambers 96and an internal accumulator 104 to maintain pressure therein. Inletchambers 94 are in fluid communication with fluid-filled sump 100 whichis located within housing 40.

[0022] Transfer clutch 76 includes a clutch hub 106 fixed via a splinedconnection 108 to pinion shaft 54, an outer drum 110 coupled via apiston housing 112 to cover assembly 82, and a clutch pack 114 having aplurality of inner clutch plates fixed (i.e., splined) to clutch hub 106that are interleaved with a plurality of outer clutch plates fixed(i.e., splined) to outer drum 110. Outer drum 110 is journalled forrotation relative to pinion shaft 54. In addition, outer drum 110 isrigidly connected (i.e., welded) to an end plate segment 116 of pistonhousing 112 which, in turn, is fixed via splined connection 118 to coverplate 86. A first exhaust passage 120 formed in housing 112 andcommunicates with a second exhaust passage 122 formed in a valve bodysection 123 of housing 40 for exhausting fluid through PWM flow controlvalve assembly 72 into a clutch chamber 124 to provide an adequatesupply of lubricating fluid for cooling and lubricating clutch pack 114.

[0023] Piston assembly 80 includes a piston chamber 126 that is formedin plate segment 116 of piston housing 112, and an actuation member orpiston 128 disposed in annular piston chamber 126. Piston 128 issupported for axial sliding movement within piston chamber 126 relativeto interleaved multi-plate clutch pack 114 for selectively applying acompressive clutch engagement force thereon, thereby transferring drivetorque from input shaft 42 to pinion shaft 54 or vise versa.

[0024] A first fluid supply passage 130 is formed in valve body section123 of housing 40 between PWM flow control valve assembly 72 and pistonhousing 112. First supply passage 130 communicates with a second supplypassage 132 formed in piston housing 112 which communicates with pistonchamber 126. An inlet passage 134 is formed in housing 40 for providingfluid communication between outlet chamber 96 of pump 78 and the inletto PWM flow control valve assembly 72. A pressure relief valve 136 isprovided in inlet passage 134 for preventing the pressure delivered tocontrol valve assembly 72 from exceeding a predetermined maximum level.

[0025] The amount of drive torque transferred is proportional to themagnitude of the clutch engagement force exerted by piston 128 on clutchpack 114 which, in turn, is a function of the fluid pressure withinpiston chamber 126. The magnitude of the control pressure (P_(C))delivered to piston chamber 126 is determined by PWM flow control valveassembly 72 which has a moveable valve element, the position of which iscontrolled by an electric control signal generated by control module 68.For example, control valve assembly 72 may be a variable force solenoidof valve using a pulse width modulation control strategy. The remainingfluid is exhaust through passages 120 and 122 at an exhaust pressure(P_(E)) which is the difference between the pump pressure P_(G)generated by gerotor pump 78 and the control pressure P_(C). As isknown, the control pressure P_(C) can be varied with precise control dueto the use of PWM valve 72.

[0026] As seen, ring seals 140 are provided for sealing piston housing112 for rotation relative to valve body section 123 of housing 40.Moreover, ring seals 142 are provided between cover plate 86 and valvebody section 123 of housing 40 to provide a fluid tight sealtherebetween. An annular chamber 144 formed in housing 40 provides fluidcommunication between outlet chambers 96 and an internal accumulator viaflow passage 145. A second flow passage 146 communicates with acircumferential chamber formed in piston housing 112 which communicateswith inlet passage 134. A thrust bearing 147 is shown between housing 40and plate segment 116 of piston housing 112.

[0027] It was previously noted that electronic control module 68monitors vehicle system information and certain hydraulic couplinginformation including wheel speed, oil sump temperature, the oil outlettemperature, and clutch pressure. In particular, the wheel speeds aredetected by four (4) wheel speed sensors 150A-105D which are disposedon, or in close proximity to, each of the vehicles' wheels. The oil sumptemperature is measured by a first temperature sensor 152 which isdisposed in oil sump 100. The oil outlet temperature is detected by asecond temperature sensor 154 that is located in proximity to clutchpack 114 in clutch chamber 124. The clutch pressure is detected by aclutch pressure sensor 156 which may be disposed in piston chamber 126or in supply passage 130.

[0028] The electronic control module 68 employs a main algorithm whichdetermines the desired clutch pressure based upon the difference infront wheel and rear wheel speed (Δ_(S)). The present inventionfunctions to modulate the clutch apply pressure through the use of PWMsolenoid valve 72 with the main algorithm control logic and closed loopcontrol. The duty cycle of the PWM control valve 72 is controlledelectronically to control the level of fluid pressure applied to piston128. Lacking any difference in speed between shafts 42 and 54, pump 78turns as a unit and creates no hydraulic flow. However, accumulator 104maintains the pump pressure at inlet 134 of control valve 72.

[0029] Upon introduction of differential speeds, the pump elements beginrelative motion and commence hydraulic flow. Pulsations in pressure dueto gerotor lobes may need to be dampened with the accumulator or othersuitable means. The PWM valve duty cycle is controlled electronically byelectronic control module 68 based upon the logic of the main algorithmand inputs from wheel speed sensors 150A-150D (ABS), pressure transducer156 and temperature sensors 152 and 154. A second pressure transducer160 can be used to provide a pressure signal to controller 68 from inletpassage 134. The wheel speed sensors are used to control the duty cycleof the PWM valve 72 that, in turn, controls the pressure being fed topiston chamber 126. They also signal controller 68 that a non-standardtire size (mini-spare) is on the vehicle so that the system can bedeactivated or the operating characteristics can be changed.

[0030] Pressure transducer 156 signals controller 68 how much torque isbeing transferred so that the logic can control the torque according topredetermined requirements. It also can be used to limit the maximumtorque transfer so that the system components can be down sized for massand cost savings. Sump temperature sensor 152 is used to compensate forfluid viscosity changes on the inlet side of pump 78. An exemplaryviscosity compensation chart is shown in FIG. 5 (labeled “viscositycompensation”) with the fluid viscosity (V) decreasing as the sump fluidtemperature (T_(S)) increases. The clutch outlet oil temperature sensor154 is used to deactivate transfer clutch 76 during thermally abusiveoperation, thereby preventing clutch damage. An exemplary clutchdeactivation curve is shown in FIG. 5 (labeled “thermal overload”).

[0031] Referring primarily now to FIG. 7, the components of drive module46 will be described. A drive pinion 220 is formed at the end of pinionshaft 54 and is meshed with a bevel ring gear 222 fixed via bolts 224 toa drive casing 226. An end cap 228 is also fixed via bolts 224 to drivecasing 226 and is supported for rotation relative to housing 40 via abearing assembly 230. A second end cap 232 is formed at the opposite endof drive casing 226 and is rotatably supported on housing 40 via abearing assembly 234. Bevel gearset 56 includes a pair of pinion gears236 rotatably supported on opposite ends of pinion shaft 238 that isnon-rotatably fixed to drive casing 226 via a retainer screw 240.Gearset 56 further includes a first side gear 242 splined for rotationwith first output shaft 58 and a second side gear 244 splined forrotation with second output shaft 60.

[0032] Second hydraulic clutch 62 includes a biasing clutch 246 and aclutch actuator 248. Biasing clutch 246 is a multi-plate clutch assemblyhaving a clutch pack 250 of alternately interleaved inner and outerclutch plates that are respectively splined to a clutch hub 252 anddrive casing 226. Hub 252 is splined to an axial hub section 254 offirst side gear 242. Clutch actuator 248 includes a fluid pump 256 and apiston assembly 258. Pump 256 is a gerotor pump assembly disposed in apump chamber formed between end cap 228 and a piston housing 260. Aneccentric outer ring 262 of gerotor pump 256 and piston housing 260 arefixed for rotation with drive casing 226 via bolts 264. Piston assembly258 is disposed in a piston chamber 266 formed in piston housing 260.Piston assembly 258 may be similar in function to that of pistonassembly 96 such that a control valve (not chown) similar to controlvalve 116 can be used. Seal rings 270 and 272 seal a piston 274 ofpiston assembly relative to piston housing 260. If piston assembly 258is similar to piston assembly 96, the hydraulic circuit shown in FIG. 5would be applicable to illustrate the operation of second hydrauliccoupling 62.

[0033] Pump 256 includes a pump ring 280 splined to first output shaft68, and a stator ring 282 disposed between pump ring 280 and eccentricring 262. The external lobes of pump ring 280 mesh with the internallobes of stator ring 282, with stator ring 282 journalled in aneccentric aperture formed in eccentric rig 262. Relative rotationbetween drive casing 226 and first output shaft 58 generates a fluidpumping action. Check valves (not shown) are retained in inlet portsformed in end cap 228 while one-way check valves (not shown) areretained in flow passages formed in piston housing 260 between theoutlet of pump 256 and piston chamber 266. These clutch valves functionsimilarly to check valves 98 and 102 described in association with firsthydraulic coupling 44. A pressure regulator valve is mounted in aby-pass passage through piston 274 to control pressurization of pistonchamber 266 so as to allow a limited amount of unrestrained inter-wheelspeed differentiation, such as during turns.

[0034] This arrangement of an in-line electronically-controlledhydraulic coupling 44 between prop shaft 78 and pinion shaft 54 permitsadaptive “on-demand” transfer of drive torque to secondary driveline 16.Thus, all-wheel drive traction control is provided when needed inresponse to a loss of traction between the front and rear drivelines.Combining the automated in-line coupling with a passively-controlledsecond hydraulic coupling 62 in drive module 46 provides “front-to-back”and “side-to-side” traction control that is well suited for use inconjunction with a secondary driveline system. It is furthercontemplated that passive hydraulic coupling 62 could be replaced withan actively-controlled hydraulic coupling similar to hydraulic coupling44 with traction control module 68 used to control speed differentiationand torque transfer between rear output shafts 58 and 60 based oncontrol algorithms and logic.

[0035] The first embodiment of drive axle assembly 34 was equipped witha positive displacement pump assembly 78 mechanically driven by relativerotation between input shaft 42 and pinion shaft 54. In contrast, FIG. 8illustrates a hydraulic circuit for hydraulic coupling 44 of drive axleassembly 34 which is now equipped with a bi-directional variabledisplacement pump 78′ which can be driven either electrically ormechanically. To reduce system power requirements, variable displacementpump 78′ can be of the vane-type used in many automatic transmissionsthat is driven at propshaft speed by input shaft 42. Accumulator 104again is used to retain pressure at the inlet of control valve assembly72 so as to assure immediate system activation upon demand. A pressurecontrol or relief valve 290 is located in the hydraulic circuit betweenaccumulator 104 and variable displacement pump 78′ and a torque limitingvalve 292 is located in the hydraulic circuit between piston chamber 126and the control pressure outlet of control valve assembly 72. Torquelimiting valve 292 is preferably an electrically-controlled solenoidvalve receiving control signals from ECU 68. However, a mechanicalpressure limiting valve is also contemplated for use with torquelimiting valve 292.

What is claimed is:
 1. A drive axle assembly for transferring drivetorque from a vehicular powertrain to a pair of wheels comprising: aninput shaft adapted to receive drive torque from the powertrain; apinion shaft; a differential drive module including a drive case drivenby said pinion shaft, a pair of axleshafts adapted for connection to thewheels, and a differential assembly interconnecting said drive case tosaid axleshafts so as to facilitate speed differentiation between saidaxleshafts; a hydraulic coupling for transferring drive torque from saidinput shaft to said pinion shaft, said hydraulic coupling including aclutch pack operably connected between said input shaft and said pinionshaft, an actuator for applying a clutch engagement force on said clutchpack in response to fluid pressure exerted thereon, and a fluid controlsystem capable of varying the fluid pressure exerted on said actuator,said fluid control system includes a source of hydraulic fluid, a pumphaving an inlet in communication with said fluid source and an outlet incommunication with said actuator, and an electric control valve disposedbetween said pump outlet and said actuator for controllably regulatingthe control pressure exerted on said actuator; speed sensors fordetermining a speed difference between said input shaft and said pinionshaft; a first temperature sensor for detecting the fluid temperature atsaid fluid source; a second temperature sensor for detecting the fluidtemperature at said clutch pack; and a traction control unit receivingspeed signals from said speed sensors and temperature signals from saidfirst and second temperature sensors and generating an electric controlsignal in response thereto, said electric control signal is supplied tosaid electric control valve for varying the control pressure as afunction of said electric control signal.
 2. The drive axle assembly ofclaim 1 wherein said traction control unit includes logic forcontrolling actuation of said control valve in response to predeterminedrelationships related to speed differences between said input shaft andsaid pinion shaft.
 3. The drive axle assembly of claim 2 wherein saidlogic further includes a sub-routine for compensating for changes influid viscosity within said fluid source based on the fluid temperaturedetected by said first temperature sensor.
 4. The drive axle assembly ofclaim 3 wherein said traction control unit is adapted to open saidcontrol valve and release said clutch pack when the fluid temperaturedetected by said second temperature sensor exceeds a predeterminedvalue.
 5. The drive axle assembly of claim 1 wherein said hydrauliccoupling is disposed within a housing which also rotatably supports saidinput shaft and pinion shaft, said pump is a gerotor pump adapted togenerate a pumping action in response to speed differentiation betweensaid pinion shaft and said input shaft, said control valve being mountedto a valvebody segment of said housing.
 6. The drive axle assembly ofclaim 1 further comprising a second hydraulic coupling operably disposedbetween said drive case and at least one of said axleshafts for limitingexcessive speed differentiation between the wheels.
 7. A drive axleassembly for transferring drive torque from a vehicular powertrain to apair of wheels, comprising: an input shaft adapted to receive drivetorque from the powertrain; a pinion shaft; a first coupling operablefor transferring drive torque from said input shaft to said pinionshaft, said first coupling including a clutch pack interconnectedbetween said input shaft and said pinion shaft, a piston supported in apiston chamber for movement in response to fluid pressure exertedtherein to apply a clutch engagement force to said clutch pack fortransferring drive torque to said pinion shaft, a fluid sump, a pumphaving an inlet in communication with said sump and an outlet, anelectric flow control valve having an inlet in communication with saidpump outlet, a first outlet in communication with said piston chamber, asecond outlet in communication with said sump, and a moveable valveelement for regulating the control pressure of hydraulic fluid suppliedthrough said first outlet to said piston chamber; speed sensors fordetecting the rotary speeds of said input shaft and said pinion shaft; afirst temperature sensor for detecting the fluid temperature at saidsump; a second temperature sensor for detecting the fluid temperature atsaid second outlet of said control valve; a controller for receivingsensor signals from said speed sensors and said first and secondtemperature sensors and generating an electric control signal inresponse thereto that is sent to said electric control valve to controlmovement of said valve element; and a drive module for transferringdrive torque from said pinion shaft to said axle shafts, said drivemodule including a drive case having a ring gear driven by a pinion gearfixed to said pinion shaft, a differential unit interconnecting saiddrive case to said axleshafts, and a second coupling operable disposedbetween said drive case and one of said axleshafts.
 8. The drive axleassembly of claim 7 wherein said second coupling includes a clutch packoperably interconnected between said drive case and said differentialunit, and a pump-actuated piston moveable relative to said clutch packin response to a pump pressure generated in response to speeddifferentiation between said drive case and one of said axleshafts. 9.The drive axle assembly of claim 7 wherein the entire assembly isdisposed with a common housing.
 10. The drive axle assembly of claim 7wherein said controller includes logic for controlling actuation of saidcontrol valve in response to predetermined relationships related tospeed differences between said input shaft and said pinion shaft. 11.The drive axle assembly of claim 10 wherein said logic further includesa sub-routine for compensating for changes in fluid viscosity withinsaid fluid source based on the fluid temperature detected by said firsttemperature sensor.
 12. The drive axle assembly of claim 11 wherein saidcontroller is adapted to open said control valve and release said clutchpack when the fluid temperature detected by said second temperaturesensor exceeds a predetermined value.
 13. The drive axle assembly ofclaim 7 wherein said hydraulic coupling is disposed within a housingwhich also rotatably supports said input shaft and pinion shaft, saidpump is a gerotor pump adapted to generate a pumping action in responseto speed differentiation between said pinion shaft and said input shaft,said control valve being mounted to a valvebody segment of said housing.14. An all-wheel drive vehicle comprising: a powertrain including anengine and a transmission; a primary driveline driven by said powertrainfor transferring drive torque to a pair of primary wheels; a powertake-off unit driven by said primary driveline; and a secondarydriveline including a drive axle assembly and a pair of secondarywheels, said drive axle assembly having an input shaft driven by saidpower take-off unit, a pinion shaft, a drive case driven by said pinionshaft, a pair of axleshafts connected to said secondary wheels, adifferential interconnecting said drive case to said axleshafts so as tofacilitate speed differentiation between said axleshafts, and ahydraulic coupling for transferring drive torque from said input shaftto said pinion shaft, said hydraulic coupling including a clutch packoperably connected between said input shaft and said pinion shaft, anactuator for applying a clutch engagement force on said clutch pack inresponse to fluid pressure exerted thereon, and a fluid control systemcapable of varying the fluid pressure exerted on said actuator, saidfluid control system includes a source of hydraulic fluid, a pump havingan inlet in communication with said fluid source and an outlet incommunication with said actuator, and an electric control valve disposedbetween said pump outlet and said actuator for controllably regulatingthe control pressure exerted on said actuator; speed sensors fordetermining a speed difference between said input shaft and said pinionshaft; a first temperature sensor for detecting the fluid temperature atsaid fluid source; a second temperature sensor for detecting the fluidtemperature at said clutch pack; and a traction control unit receivingspeed signals from said speed sensors and temperature signals from saidfirst and second temperature sensors and generating an electric controlsignal in response thereto, said electric control signal is supplied tosaid electric control valve for varying the control pressure as afunction of said electric control signal.
 15. The all-wheel drivevehicle of claim 14 wherein said traction control unit includes logicfor controlling actuation of said control valve in response topredetermined relationships related to speed differences between saidinput shaft and said pinion shaft.
 16. The all-wheel drive vehicle ofclaim 15 wherein said logic further includes a sub-routine forcompensating for changes in fluid viscosity within said fluid sourcebased on the fluid temperature detected by said first temperaturesensor.
 17. The all-wheel drive vehicle of claim 16 wherein saidtraction control unit is adapted to open said control valve and releasesaid clutch pack when the fluid temperature detected by said secondtemperature sensor exceeds a predetermined value.
 18. The all-wheeldrive vehicle of claim 14 further comprising a second hydraulic couplingoperably disposed between said drive case and at least one of saidaxleshafts for limiting excessive speed differentiation between thewheels.
 19. An all-wheel drive vehicle comprising: a powertrainincluding an engine and a transmission; a primary driveline driven bysaid powertrain for transferring drive torque to a pair of primarywheels; a power take-off unit driven by said primary driveline; asecondary driveline including a drive axle assembly and a pair ofsecondary wheels, said drive axle assembly having an input shaft drivenby said power take-off unit, a pinion shaft, a first torque couplingoperable for transferring drive torque from said input shaft to saidpinion shaft, said first torque coupling including a clutch packinterconnected between said input shaft and said pinion shaft, a pistonsupported in a piston chamber for movement in response to fluid pressureexerted thereon to apply a clutch engagement force to said clutch packfor transferring drive torque to said pinion shaft, a pump having aninlet in communication with a sump and an outlet, and an electric flowcontrol valve, said control valve having an inlet in communication withsaid pump outlet, a first outlet in communication with said pistonchamber, a second outlet in communication with said sump, and a valveelement that is moveable for regulating the control pressure ofhydraulic fluid supplied through said first outlet to said pistonchamber; speed sensors for detecting the rotary speeds of said inputshaft and said pinion shaft; a first temperature sensor for detectingthe temperature of fluid in said sump, a second temperature sensor fordetecting the fluid temperature at said second outlet of said controlvalve; a controller for receiving sensor signals from said speed sensorsand said first and second temperature sensors and generating an electriccontrol signal in response thereto that is sent to said electric controlvalve to control movement of said valve element, and a drive module fortransferring drive torque from said pinion shaft to said secondarywheels, said drive module including a drive case having a ring geardriven by a pinion gear fixed to said pinion shaft, a differentialinterconnecting said drive case to said secondary wheels, and a secondtorque coupling operable disposed between said drive case and one ofsaid secondary wheels.
 20. The all-wheel drive vehicle of claim 19wherein said second torque coupling includes a clutch pack operablyinterconnected between said drive case and said differential, and apump-actuated piston moveable relative to said clutch pack in responseto a pump pressure generated in response to speed differentiationbetween said drive case and one of said secondary wheels.
 21. Theall-wheel drive vehicle of claim 19 wherein said controller includeslogic for controlling actuation of said control valve in response topredetermined relationships related to speed differences between saidinput shaft and said pinion shaft.
 22. The all-wheel drive vehicle ofclaim 21 wherein said logic further includes a sub-routine forcompensating for changes in fluid viscosity within said fluid sourcebased on the fluid temperature detected by said first temperaturesensor.
 23. The all-wheel drive vehicle of claim 22 wherein saidcontroller is adapted to open said control valve and release said clutchpack when the fluid temperature detected by said second temperaturesensor exceeds a predetermined value.